Sequestering Ability of Phytate toward Biologically and

Jul 30, 2012 - In general, phytate results in a quite good sequestering agent toward all three cations in the whole investigated pH range, but the ord...
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Sequestering Ability of Phytate toward Biologically and Environmentally Relevant Trivalent Metal Cations† Clemente Bretti, Rosalia Maria Cigala, Gabriele Lando, Demetrio Milea, and Silvio Sammartano* Dipartimento di Chimica Inorganica, Chimica Analitica e Chimica Fisica, Università di Messina, Viale Ferdinando Stagno d’Alcontres 31, I-98166 Messina (Vill. S. Agata), Italy ABSTRACT: Quantitative parameters for the interactions between phytate (Phy) and Al3+, Fe3+, and Cr3+ were determined potentiometrically in NaNO3 aqueous solutions at I = 0.10 mol L−1 and T = 298.15 K. Different complex species were found in a wide pH range. The various species are partially protonated, depending on the pH in which they are formed, and are indicated with the general formula MHqPhy (with 0 ≤ q ≤ 6). In all cases, the stability of the FeHqPhy species is several log K units higher than that of the analogous AlHqPhy and CrHqPhy species. For example, for the MH2Phy species, the stability trend is log K2 = 15.81, 20.61, and 16.70 for Al3+, Fe3+, and Cr3+, respectively. The sequestering ability of phytate toward the considered metal cations was evaluated by calculating the pL0.5 values (i.e., the total ligand concentration necessary to bind 50% of the cation present in trace in solution) at different pH values. In general, phytate results in a quite good sequestering agent toward all three cations in the whole investigated pH range, but the order of pL0.5 depends on it. For example, at pH 5.0 it is pL0.5 = 5.33, 5.44, and 5.75 for Fe3+, Cr3+, and Al3+, respectively (Fe3+ < Cr3+ < Al3+); at pH 7.4 it is pL0.5 = 9.94, 9.23, and 8.71 (Al3+ < Cr3+ < Fe3+), whereas at pH 9.0 it is pL0.5 = 10.42, 10.87, and 8.34 (Al3+ < Fe3+ < Cr3+). All of the pL0.5 values, and therefore the sequestering ability, regularly increase with increasing pH, and the dependence of pL0.5 on pH was modeled using some empirical equations. KEYWORDS: iron(III), chromium(III), aluminum(III), phytate complexes, sequestering ability, speciation

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INTRODUCTION Phytic acid [1,2,3,4,5,6-hexakis(dihydrogen phosphate) myoinositol] (Phy) and its salts (the phytates, sometimes also denoted phytines2) are widely present in nature, mostly in seeds and grains, but are ubiquitous in all eukaryotic cells,3 where they play several key roles.2−18 In the past decades, interest in the study of phytic acid has been growing considerably, from both environmental and biological points of view, as well as from the technological and industrial perspective. New biological functions and technological applications are being continuously discovered, so that any attempts at their description and classification in this context would be limiting and probably outside the aims of this paper: for a comprehensive and exhaustive picture of phytate properties one may refer to some past and recent reviews and books published on these topics.2−18 However, it is important to stress here that most of these properties are strictly related to the ability of phytate to form quite strong soluble and sparingly soluble complexes with various proteins, carbohydrates, other organic ligands, and several metal and organometal cations, influencing their (bio)availability and their activity (see, e.g., references in refs 2−18). This means that the chemical speciation (unambiguously defined by IUPAC as the distribution of an element among defined chemical species in a system19) of both phytate and all of these substances is mutually and deeply affected by their strong interactions. For this reason, several efforts were addressed during the years to the determination of various formation thermodynamic parameters (e.g., activity coefficients, stability constants, entropy and enthalpy changes, solubility) of different phytate complexes in aqueous solution, because they represent the basis © 2012 American Chemical Society

for a correct speciation study of any substances in different natural fluids (e.g., natural waters, soil solutions, biological fluids). Some of these results have been recently reviewed by this research group,20 but new and more accurate investigations on the complexation by phytate of different metal and organometal cations, as well as other organic molecules, are being continuously published (see, e.g., refs 1 and 21−34). In this context, the quantitative studies on the binding ability of phytic acid toward trivalent metal cations are very few (see, e.g., refs 28, 31, and 34−37), although several biological functions, the nutritional properties of some foods, and/or numerous environmentally relevant processes are strictly dependent on the extent of these interactions. In fact, this aspect is particularly relevant from both the agricultural/botanical and the dietary/ medical points of view: for example, it is well-known that iron intake via plant food is strongly limited by its chelation by phytate, causing iron deficiency anemia in populations sustained by staple food crops.38,39 It is also demonstrated that phytate is deeply involved in the better adaptation of plants to grow in low-P−high-Al acid soils.40 In this light, this paper reports the results of a quantitative study on the speciation and the sequestering ability of phytate in the presence of three environmentally and biologiclally relevant trivalent metal cations, namely, Fe3+, Al3+, and Cr3+. Speciation studies were performed to model the behavior of the M3+/Phy systems in different conditions as a function of pH and, when possible, a Received: Revised: Accepted: Published: 8075

May 8, 2012 July 27, 2012 July 30, 2012 July 30, 2012 dx.doi.org/10.1021/jf302007v | J. Agric. Food Chem. 2012, 60, 8075−8082

Journal of Agricultural and Food Chemistry

Article

Due to the small dehydration rate constants for trivalent metal cations such as those here considered, the full equilibration of complex species formed may take very long time. That is why particular attention has been paid to this last aspect, verifying the equilibrium state during titrations by some usual precautions44 such as checking the time required to reach equilibrium and performing back-titrations. Finally, for measurements performed at low ionic strengths, the contribution of the ligand to the total ionic strength must be considered. At the highest Phy concentrations, this contribution was taken into account by giving appropriate weights to the results obtained, because the used computer programs can deal with measurements at different ionic strengths and can also determine the ionic strength changes within a single titration. Ligand Competition and Batch Measurements. In addition to the above-cited potentiometric titrations, further measurements were also performed by different procedures, to verify the reliability of the results obtained. In fact, two difficulties arise when dealing with the systems investigated in the present work: (i) the above-mentioned slow complexation kinetics of some M3+ species and (ii) the high stability of phytate/M(III) species. The first problem has been already discussed. Concerning the last aspect, it must be emphasized that it seriously limits the use of ISE-H+ potentiometric titrations for the determination of these species, as well detailed, for example, by Delgado et al.:45 “The determination of very high values of stability constants has led usually to critical problems, because in most cases the pH-potentiometric techniques (titrations using a strong base to neutralize the protons released by the formation of the complex and followed by a couple of glass/reference electrodes) cannot be applied, as the complex is already formed at very low pH values” and protons are already displaced from the ligand. This generally occurs in the case of relatively stable species, characterized by formation constants >1020 (as in the case of the Fe3+/Phy and Al3+/Phy systems), where we cannot observe an equilibrium of the proton during the titration, because the reaction in eq 1

critical comparison of the thermodynamic results obtained in this paper with literature findings is also reported. The sequestering ability of phytic acid toward the studied metal cations was also quantified by the calculation of the pL0.5 values (i.e., the total ligand concentration necessary to bind 50% of the cation present in trace in solution, generally cM < 10−9 mol L−1)41 at different pH values.

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MATERIALS AND METHODS

Chemicals. Phytic acid solutions were prepared by weighing the dipotassium salt (K2H10Phy, purity > 95%, main impurity is water) and passing it over a strong cationic exchange resin (Dowex 50W X 8) in H+ form. The concentration was checked potentiometrically by alkalimetric titrations, and the absence of potassium was established by flame emission spectrometry, always being lower than the LOQ (limit of quantification), Cr3+ > Al3+. As an example, for the MH2Phy species it is log K2 = 20.61, 16.70, and 15.51 for Fe3+, Cr3+, and Al3+, respectively. In Figure 2, the speciation diagram of the Cr3+/Phy system is reported. In these conditions (cCr = 1 mmol L−1 and cPhy = 3

Figure 3. Speciation diagram of Al3+/Phy system; fraction of Al versus pH. Experimental conditions: cPhy = 3 mmol L−1; cAl = 1 mmol L−1; I = 0.10 mol L−1 in NaNO3aq, T = 298.15 K. Species: 1, AlPhy; 2, AlHPhy; 3, AlH2Phy; 4, AlH3Phy; 5, AlH4Phy; 6, AlH5Phy; 7, Al (charges omitted for simplicity); Σ, sum of all AlHqPhy species.

the formation of sparingly soluble species was observed) and higher pH values were also reached during the measurements, depending on the different experimental conditions. In particular, the latter (higher pH) was also the case of the ligand competition measurements, in which the addition of another strong ligand such as EDTA hampered the formation of precipitate, allowing the investigation of a wider pH range. For example, the usual pH values reached in the simple M3+/ Phy measurements at low M3+/Phy ratios were pH ∼6.0 and ∼8.0 for Fe3+ and Al3+, respectively, whereas pH ∼9.0 and ∼10.0 were reached in the Fe3+/Phy/EDTA and Al3+/Phy/ EDTA systems. A speciation diagram of the Fe3+/Phy/EDTA system is reported in Figure 4 in the following experimental conditions: cPhy = 2 mmol L−1, cEDTA = 1 mmol L−1, cFe = 2 mmol L−1. Although EDTA is present in lower concentration than phytate, the Fe(EDTA) and FeOH(EDTA) species are

Figure 2. Speciation diagram of the Cr3+/Phy system; fraction of Cr versus pH. Experimental conditions: cPhy = 3 mmol L−1; cCr = 1 mmol L−1; I = 0.10 mol L−1 in NaNO3aq, T = 298.15 K. Species: 1, CrPhy; 2, CrHPhy; 3, CrH2Phy; 4, CrH3Phy; 5, CrH4Phy; 6, CrH5Phy; 7, CrH6Phy; 8, Cr (charges omitted for simplicity); Σ, sum of all CrHqPhy species.

mmol L−1), all of the CrHqPhy species show high formation percentages, dominating chromium speciation. For example, the CrPhy, CrH2Phy, and CrH4Phy species represent ∼95, ∼75, and ∼80% of total chromium (at pH >8, ∼6.5, and ∼4, respectively). On the contrary, the CrHPhy and the CrH6Phy species reach ∼5 and ∼20% at pH ∼7 and ∼2.5, respectively. Quite different trends are observed for the formation of the AlHqPhy species in the same conditions, as shown in the speciation diagram reported in Figure 3. In this case, slight differences occur both in the species formation percentages and in the pH range over which they are formed. For example, it has already been shown that the CrH4Phy species reaches a maximum of 95% at pH ∼4.0−4.5, whereas the analogue AlH4Phy reaches a maximum of ∼70% at pH ∼3.0−3.5. The same holds for CrHPhy, formed in negligible amounts at pH ∼7.0, whereas the AlHPhy at pH >6.0 is already >50%. All of the speciation diagrams reported are plotted up to pH ∼8.5. However, as already stated, lower (in the cases in which

Figure 4. Speciation diagram of the Fe3+/Phy/EDTA system; fraction of Fe versus pH. Experimental conditions: cPhy = 2 mmol L−1; cEDTA = 1 mmol L−1; cFe = 2 mmol L−1; I = 0.10 mol L−1 in NaNO3aq, T = 298.15 K. Species: 1, FePhy; 2, FeHPhy; 3, FeH2Phy; 4, FeH3Phy; 5, FeH4Phy; 6, FeOH(EDTA); 7, Fe(EDTA) (charges omitted for simplicity). 8078

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both formed in comparable formation percentages: ∼50% Fe3+ is present as Fe(EDTA) at pH 8, whereas the FeHqPhy species, formed in all the investigated pH range, reach ∼50% as FePhy at pH > 7.0 and ∼40% as FeH2Phy and FeH3Phy pH ∼5.0 and ∼3.5, respectively. Lower formation percentages are observed for the FeH4Phy (∼20% at pH ∼2.5) and the FeHPhy species (∼15% at pH ∼6.0), whereas the formation of the Fe(OH)Phy species is not observed in those experimental conditions. The speciation diagram of Figure 4 is also useful to assess the distribution of Fe3+ in conditions that may be frequently observed in foods or soils. In fact, chlorosis in plants and other problems caused by iron deficiency are frequently treated by amending soils with FeEDTA or other iron chelates,59,60 and NaFeEDTA has been recommended as a food additive for iron fortification, particularly in the cases of high-phytate diets.61 As a consequence, iron availability may be modified by the simultaneous presence of phytate and EDTA. The speciation diagrams in Figures 1−4, obtained by the ES4ECI program using the conditional stability constants reported in Table 1 (with the appropriate protonation and hydrolysis constants), are just four examples of the possible conditions in which the investigated metal cations can be present with phytate. Of course, the stability constants provided can be used to draw the speciation diagrams in other possible systems with different phytate and metal concentrations. Sequestering Ability of Phytate and Dependence on pH. The above-cited examples are just two of the numerous cases where speciation studies are useful. In fact, one of its most important applications is the estimation of the sequestering ability of a given ligand toward one or more metal cations or of one or more ligands toward a given cation. This is a key information, for example, in remediation studies (assisted or not) of polluted sites or in the prediction of the availability of some components in real systems. To this aim, the simple analysis of single sets of stability constants of metal/ligand systems is not always sufficient to assess the global binding (sequestering) ability of a ligand toward a given cation in real conditions, owing to the difficulties regarding, for example, the different number and/or nature of complexes formed by single ligands. Moreover, other factors influence the formation yields of the species, such as the solution conditions, the acid−base properties of the cation and the ligand (because both hydrolysis and protonation reactions are competitive with respect to the formation reaction), and the competition between other metals and ligands simultaneously present in natural systems. Because of this, two metal−ligand systems may show the same formation percentages (in given conditions), even with different formation constants. This problem can be overcome by the calculation of pL0.5 (also called pL50), an empirical parameter (proposed by our group in recent years) that, once the experimental conditions (ionic strength, ionic medium, temperature, pH) are fixed, can give an objective representation of the sequestering ability of a ligand (L) toward a metal ion (M). A detailed description of the method is given, for example, in ref 41. In practice, using any computer program able to calculate the free concentrations of various species in solution (i.e., speciation diagrams obtained by, e.g., ES4ECI), the mole fractions (x) (or the percentages) of a cation complexed by a ligand (phytate in our case) can be determined at various total ligand concentrations (as −log cL) and can be plotted as a function of it at fixed pH, ionic strength, and temperature values. The result is a diagram (sequestration diagram)

representing a sigmoid curve (similar to a dose−response curve) with asymptotes of 1 (or 100 if expressed in percentage) for pL → −∞ and 0 for pL → +∞ ⎡ ⎤ 1 x=⎢ ⎥ (pL − pL ) 0.5 ⎦ ⎣ 1 + 10

(8)

where the parameter pL0.5 represents the total ligand concentration necessary to sequester 50% of the metal ion present in trace in solution. Therefore, the higher the pL0.5 is, the stronger is the sequestering ability toward a given cation. In general, when a sequestration diagram is drawn, x values are dependent on the total metal concentration cM. However, when cM is low enough (generally cM < 10−9 mol L−1), x and the relative sequestration diagrams become independent of it, as well as the corresponding pL0.5. Of course, several other parameters have been proposed for the same purpose,62 but here it is important to stress that, different from most other methods, all of the side interactions occurring in the system (metal hydrolysis, ligand protonation, interactions with other components) are taken into account in the speciation model used in the calculation of pL0.5, but are excluded from its estimation and do not give any contribution. In this way, the pL0.5 value quantifies the sequestering power of a ligand, ‘‘cleaned’’ from all competitive reactions, simplifying comparisons. In fact, this conditional parameter is particularly useful to make comparisons among systems in different conditions or with different speciation (e.g., ligands and metals with different acid−base properties or with a different number of metal complexes formed, as well as the cases where polynuclear species are formed). In these cases, the simple analysis of the stability constants is not sufficient to give an immediate idea of the sequestration of a cation by different ligands. For example, concerning the cases cited in the previous paragraph, where it is important to establish which ligand among EDTA and phytate is a stronger chelant toward Fe3+, from the simple comparison of the stability constants of the FeL species it could be expected that the former is weaker than the latter (log K = 25.1 and 28.5, for Fe(EDTA) and FePhy, respectively). Nevertheless, by looking at the sequestering ability, the conclusion is different and supports the observation that Fe-EDTA is successfully employed as iron supplier also in the cases where phytate is present. In fact, in all of the investigated pH range, EDTA shows a greater sequestering ability than Phy: at pH 7.4, for example, it is pL0.5 = 13.5 and 9.9, for EDTA and Phy, respectively. These values indicate that both ligands are good sequestering agents toward Fe3+, but their difference (∼3.5 log units) allows Fe3+ to be preferentially bound to EDTA than phytate. This depends, of course, on the different acid−base properties of the two ligands, which deprotonate at different pH values and for which information is not immediately accessible from the simple analysis of the log KML values. For this reason, the analysis of the pL0.5 is a very interesting tool, also for nonchemists, to make fast comparisons. Analogously, from the simple analysis of the stability constants of the three investigated systems, it is not possible to make inferences about the greater sequestering ability of phytate toward Al3+, Fe3+, or Cr3+, especially in different pH conditions. In fact, the pL0.5 values calculated at different pH values (reported in Table 2) show that phytic acid is a quite good sequestering agent toward these cations over the entire pH range. However, phytate sequesters preferentially one cation or another, depending on the pH. For example, in the 8079

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Table 2. pL0.5 Valuesa for M3+/Phy Systems Calculated at Different pH Values

some empirical relationships to be used with predictive purposes. Simple equations have been proposed for the calculation of pL0.5 as a function of pH, ionic strength, temperature, and the concentration of other competing ligands (see, e.g., refs 1, 29, 33, and 41). For example, pL0.5 values in Table 2 generally increase with increasing pH and, in the case of Cr3+, the function is

pL0.5

a

pH

Al3+

Fe3+

Cr3+

4.0 5.0 6.0 7.4 8.1 9.0

5.75 7.21 7.41 8.71 8.91 8.34

5.33 6.73 8.10 9.94 10.44 10.42

5.44 6.82 7.80 9.23 10.32 10.87

pL0.5 ( ±0.1) = 1.21 + 1.09(pH)

(with a correlation coefficient, r = 0.996) in the pH range 4.0 ≤ pH ≤ 9.0. For the other two systems, we calculated, at 4.0 ≤ pH ≤ 8.0

± 0.1 (95% CI).

pL0.5 ( ±0.1) = 0.35 + 1.27(pH)

pH range ∼6.0−8.0, the pL0.5 values of the Fe3+/Phy system are higher than the Cr3+/Phy, in turn higher than the Al3+/Phy system, whereas the trend Al3+/Phy > Cr3+/Phy > Fe3+/Phy is observed at pH 9.0, when the competition of hydrolytic species is more relevant for both Al3+ and Fe3+ than Cr3+, the sequestering ability of phytate is greater toward the latter than the former. As an example, the comparison of the sequestration diagrams for the three M3+/Phy systems is reported at pH 7.4 in Figure 5. However, the different behavior

(r = 0.996)

and pL0.5 ( ±0.2) = 3.07 + 0.75(pH)

(r = 0.966)

for Fe3+ and Al3+, respectively. This kind of empirical relationship makes the pL0.5 a very helpful tool, especially when the information about stabilities is incomplete: these equations make it possible to estimate with good approximation the sequestering ability of a ligand toward a metal cation in conditions different from the experimental. As already evidenced for other Mz+/Phy systems,29,47,58,63 also the formation constants of phytate metal complexes usually show some regular trends, which can be used to estimate the stability of other species not experimentally determined, but which can be formed in other contexts. In Figure 6, the

Figure 5. Sequestration diagram of phytate toward different trivalent metal cations at pH 7.4, I = 0.10 mol L−1 in NaNO3aq, T = 298.15 K. Molar fraction of the complexed metal versus −log (cPhy/mol L−1). Curves: 1, Al3+; 2, Cr3+; 3, Fe3+.

observed at various pH values is an indication of the possibility for phytate to selectively sequester one of these cations in the presence of the others, by simply varying the acidity conditions of the system. This “selective sequestration” is one of the various aspects in support of the use of phytate as a commercial sequestering agent. In addition, it shows at least three other great advantages with respect to other classical chelating agents: (1) it is essentially nontoxic and already present in nature in great amounts; (2) it has a very low cost (because it can be extracted from a wide number of vegetables in high percentages); and, thanks to its acid base properties, (3) it shows an almost constant high sequestering ability over a very wide pH range, so that it could be employed in very different conditions. Empirical (Predictive) Relationships. Another aspect that makes the pL0.5 interesting is that, although its values are dependent on the selected conditions, they very often show regular trends that may be exploited for the formulation of

Figure 6. Log Kq values versus q: dependence of the conditional stability constants (log Kq) of MHqPhy complexes on the number of protons (q) of the species for the Al3+/Phy (squares), Fe3+/Phy (circles), and Cr3+/Phy (triangles) systems.

dependence of the stability constants (log Kq) of different MHqPhy species on the number of protons (q) of the complex is reported for all three investigated cations. The log Kq values were fitted to a first-degree (linear, dashed line) and a seconddegree (straight line) polynomial curve, obtaining quite high correlation coefficients in all cases. The refined parameters for the three systems and the two fits are reported in Table 3 together with the corresponding correlation coefficients. As is obvious, the polynomial fit is better than the linear and can be used for the estimation of unknown stability constants. 8080

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(12) Fox, C. H.; Eberl, M. Phytic acid (IP6), novel broad spectrum anti-neoplastic agent: a systematic review. Complement. Ther. Med. 2002, 10 (4), 229−234. (13) Lopez, H. W.; Leenhardt, F.; Coudray, C.; Remesy, C. Minerals and phytic acid interactions: is it a real problem for human nutrition? Int. J. Food Sci. Technol. 2002, 37 (7), 727−739. (14) Reddy, N. R.; Sathe, S. K. Food Phytates; CRC Press: Boca Raton, FL, 2002. (15) Shamsuddin, A. M. Anti-cancer function of phytic acid. Int. J. Food Sci. Technol. 2002, 37 (7), 769−782. (16) Konietzny, U.; Jany, K. D.; Greiner, R. Phytate − an undesiderable constituent of plant-based foods? J. Ernaehrungsmed. 2006, 8 (3), 18−28. (17) Kumar, V.; Sinha, A. K.; Makkar, H. P. S.; Becker, K. Dietary roles of phytate and phytase in human nutrition: a review. Food Chem. 2010, 120 (4), 945−959. (18) Ali, M.; Shuja, M. N.; Zahoor, M.; Qadri, I. Phytic acid: how far have we come? Afr. J. Biotechnol. 2010, 9 (11), 1551−1554. (19) Templeton, D. M.; Ariese, F.; Cornelis, R.; Danielsson, L. G.; Muntau, H.; van Leeuwen, H. P.; Lobinski, R. Guidelines for terms related to chemical speciation and fractionation of elements. Definitions, structural aspects, and methodological approaches. Pure Appl. Chem. 2000, 72 (8), 1453−1470. (20) Crea, F.; De Stefano, C.; Milea, D.; Sammartano, S. Formation and stability of phytate complexes in solution. Coord. Chem. Rev. 2008, 252, 1108−1120. (21) Crea, F.; Crea, P.; De Stefano, C.; Milea, D.; Sammartano, S. Speciation of phytate ion in aqueous solution. Protonation in CsClaq at different ionic strengths and mixing effects in LiClaq + CsClaq. J. Mol. Liq. 2008, 138, 76−83. (22) Crea, F.; De Stefano, C.; Porcino, N.; Sammartano, S. Sequestering ability of phytate towards protonated BPEI and other polyammonium cations in aqueous solution. Biophys. Chem. 2008, 136, 108−114. (23) Crea, P.; De Stefano, C.; Milea, D.; Sammartano, S. Formation and stability of mixed Mg2+/Ca2+/phytate species in seawater media. Consequences on ligand speciation. Mar. Chem. 2008, 112 (3−4), 142−148. (24) Heighton, L.; Schmidt, W. F.; Siefert, R. L. Kinetic and equilibrium constants of phytic acid and ferric and ferrous phytate derived from nuclear magnetic resonance spectroscopy. J. Agric. Food Chem. 2008, 56 (20), 9543−9547. (25) Torres, J.; Veiga, N.; Gancheff, J. S.; Dominguez, S.; Mederos, A.; Sundberg, M.; Sanchez, A.; Castiglioni, J.; Diaz, A.; Kremer, C. Interaction of myo-inositol hexakisphosphate with alkali and alkaline earth metal ions: spectroscopic, potentiometric and theoretical studies. J. Mol. Struct. 2008, 874 (1−3), 77−88. (26) Crea, F.; De Stefano, C.; Milea, D.; Sammartano, S. Speciation of phytate ion in aqueous solution. Thermodynamic parameters for zinc(II) sequestration at different ionic strengths and temperatures. J. Solution Chem. 2009, 38 (1), 115−134. (27) De Carli, L.; Schnitzler, E.; Ionashiro, M.; Szpoganicz, B.; Rosso, N. D. Equilibrium, thermoanalytical and spectroscopic studies to characterize phytic acid complexes with Mn(II) and Co(II). J. Braz. Chem. Soc. 2009, 20, 1515−1522. (28) Quirrenbach, H. R.; Kanumfre, F.; Rosso, N. D.; Carvalho, M. A. Behaviour of phytic acid in the presence of iron(II) and iron(III). Cienc. Tecnol. Aliment. 2009, 29 (1), 24−32. (29) Cigala, R. M.; Crea, F.; De Stefano, C.; Lando, G.; Milea, D.; Sammartano, S. Electrochemical study on the stability of phytate complexes with Cu2+, Pb2+, Zn2+, and Ni2+: a comparison of different techniques. J. Chem. Eng. Data 2010, 55, 4757−4767. (30) Cigala, R. M.; Crea, F.; Lando, G.; Milea, D.; Sammartano, S. Solubility and acid-base properties of concentrated phytate in selfmedium and in NaCl(aq) at T = 298.15 K. J. Chem. Thermodyn. 2010, 42, 1393−1399. (31) Crea, F.; De Stefano, C.; Milea, D.; Sammartano, S. Thermodynamic data for lanthanoid(III) sequestration by phytate at different temperatures. Monatsh. Chem. 2010, 141 (5), 511−520.

Table 3. Empirical Parameters of the Equations for the Dependence of the Conditional Stability Constantsa of MHqPhy Complexes on the Number of Protons of the Species, in NaNO3aq at I = 0.10 mol L−1 and T = 298.15 K M3+

a

log Kq = a + bq Al 22.4 Fe 29.1 Cr 22.9 log Kq = a + bq + cq2 Al 23.1 Fe 28.5 Cr 22.2

b

c

−3.17 −4.59 −3.25 −4.00 −3.46 −2.51

rb 0.996 0.996 0.992

0.14 −0.28 −0.12

0.999 0.999 0.993

a

log Kq refers to equilibrium: M + HqL = MHqL, charges omitted for simplicity. bCorrelation coefficient.

Nevertheless, the linear fit is more reliable if the order of magnitude of the experimental errors made during the determination of the stability constants is taken into account.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +39-090-6765749. Fax: +39-090-392827. Author Contributions †

Speciation of phytate ion in aqueous solution. Last contribution to this series: Cigala, R. M.; Crea, F.; De Stefano, C.; Lando, G.; Milea, D.; Sammartano, S. Thermodynamics of binary and ternary interactions in the tin(II)/phytate system in aqueous solutions, in the presence of Cl− or F−. J. Chem. Thermodyn. 2012, 51, 88−96. Funding

We thank the University of Messina for partial financial support. Notes

The authors declare no competing financial interest.

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REFERENCES

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